Other Galaxies: Hubble supersedes Shapley Edwin Hubble identified single stars in the Andromeda nebula (“turning” it into a galaxy) Measured the distance to Andromeda to be 1 million Ly (modern value: 2.2 mill. Ly) Conclusion: it is 20 times more distant than the milky way’s radius  Extragalacticity!  Shapley’s theory falsified!

Q: How many galaxies are there? Hubble Deep Field Project 100 hour exposures over 10 days Covered an area of the sky about 1/100 the size of the full moon Probably about 100 billion galaxies visible to us!

About 1,500 galaxies in this patch alone Angular size ~ 2 minutes of arc

Other Galaxies there are ~ 100 billion galaxies in the observable Universe measure distances to other galaxies using the period-luminosity relationship for Cepheid variables Type I supernovae also used to measure distances Predictable luminosity – a standard candle Other galaxies are quite distant Andromeda (M31), a nearby (spiral) galaxy, is 2 million light-years away and comparable in size to Milky Way “Island universes” in their own right

Q: How does our galaxy look like from the outside? Probably like others, so observe them!

Hubble Classification Scheme Edwin Hubble (~1924) grouped galaxies into four basic types: Spiral Barred spiral Elliptical Irregular There are sub-categories as well

Spirals (S) All have disks, bulges, and halos Type Sa: large bulge, tightly wrapped, almost circular spiral arms Type Sb: smaller bulge, more open spiral arms Type Sc: smallest bulge, loose, poorly defined spiral arms

Barred Spirals (SB) Possess an elongated “bar” of stars and interstellar mater passing through the center

Elliptical (E) No spiral arms or clear internal structure Essentially all halo Vary in size from “giant” to “dwarf” Further classified according to how circular they are (E0–E7) Left E0, right E3

S0/SB0 Intermediate between E7 and Sa Ellipticals with a bulge and thin disk, but no spiral arms Not an evolutionary diagram!

Q: How do we know we live in a Spiral Galaxy? After correcting for absorption by dust, it is possible to plot location of O- and B- (hot young stars) which tend to be concentrated in the spiral arms Radio frequency observations reveal the distribution of hydrogen (atomic) and molecular clouds Evidence for galactic bulge spiral arms Spiral

Rotation of the Galaxy Stars near the center rotate faster; those near the edges rotate slower (Kepler) The Sun revolves at about 250 km/sec around the center Takes 200-250 million years to orbit the galaxy – a “galactic year” observed by measuring the velocities of stars near the Sun; those nearer the center of the Galaxy are passing us by, those further out are falling behind. Similar differential rotation observed in other nearby galaxies, such as Andromeda. The halo stars also orbit the galactic center, but not in as organized a way.

How do spiral arms persist?  Why don’t the “curl up”?

“Spiral Density Waves” A spiral compression wave (a shock wave) moves through the Galaxy Triggers star formation in the spiral arms Explains why we see many young hot stars in the spiral arms

Density (Shock) Waves

The Mass of the Galaxy Can be determined using Kepler’s 3rd Law Solar System: the orbital velocities of planets determined by mass of Sun Galaxy: orbital velocities of stars are determined by total mass of the galaxy contained within that star’s orbit Two key results: large mass contained in a very small volume at center of our Galaxy Much of the mass of the Galaxy is not observed consists neither of stars, nor of gas or dust extends far beyond visible part of our galaxy (“dark halo”)

Galaxy Masses Rotation curves of spiral galaxies comparable to milky way Masses vary greatly

The Missing Mass Problem Dark Matter is dark at all wavelengths, not just visible light The Universe as a whole consists of up to 25% of Dark Matter!  Strange! What is it? Brown dwarfs? Black dwarfs? Black holes? Neutrinos? Other exotic subatomic particles? Actually: Most of the universe (70%) consists of Dark Energy  Even stranger!

Missing Mass Problem Actual data Hypothetical Keplerian motion Keplerian Motion: more distance from center  less gravitational pull  slower rotational speed

Galaxy Formation Not very well understood More complicated than stellar formation, and harder to observe Formation of galaxies begins after Big Bang Different than star formation because galaxies may collide and merge 21

Galaxy Formation Galaxies are probably built up by mergers Contrast to break up of clouds in star formation Our own Milky Way is eating up the neighboring Sagittarius Dwarf Galaxy

Galaxy Mergers Start with high density of small proto-galaxies Galaxies merge and turn into bigger galaxies Actual photo (HST): lots of small galaxies

Galaxy Interaction Galaxy Collision: NGC2207 vs. IC2163 Smaller galaxy, IC 2163, swings by the larger NGC 2207 in a counter-clockwise motion Closest approach 40 million years ago. These two are gravitationally stuck together! Galaxy Collision: NGC2207 vs. IC2163 24

Collision between NGC 4038 and NGC 4039 25

The Tully-Fisher Relation A relation between the rotation speed of a spiral galaxy and its luminosity The more mass a galaxy has the brighter it is  the faster it rotates  the wider the spectral lines are Measuring rotation speed allows us to estimate luminosity; comparing to observed (apparent) brightness then tells us the distance

Beyond the Galactic Scale – Clusters of Galaxies The Local Group The Virgo Cluster

Superclusters

Beyond Superclusters Strings, filaments, voids Reflect structure of the universe close to the Big Bang Largest known structure: the Great Wall (70 Mpc  200 Mpc!)